For many medical applications, such as dermatological applications, a laser is not required to have the highest possible beam quality and accordingly is not required to operate in a single fundamental mode. For these applications, efficiency, stability and compactness are important characteristics of a laser. Multimode operation of a laser typically provides higher efficiency than single-mode operation.
One laser preferred for dermatological applications is a flashlamp-pumped solid-state laser having a resonator including an Er:YAG gain medium. Such a laser is typically arranged to provide laser-radiation having a wavelength of 2.94 micrometers (.mu.m). A preferred design of such a laser includes a symmetrical resonator formed between two plane (flat) mirrors. This can be defined as a flat-flat resonator. One of the mirrors is a maximally-reflecting mirror. The other mirror is a partially-transmitting mirror serving as an output-coupling mirror.
The gain medium is in the form of a rod of circular cross-section and is symmetrically disposed between the mirrors. The ends of the rod have a concave radius of curvature. The radius of curvature is the same at both ends of the rod. The concave radius of the rod ends serves to partially offset a positive thermal-lensing effect in the rod, the thermal-lensing effect resulting from absorption of pump-light. This resonator arrangement produces a multimode laser-beam having the same width on each end of the resonator mirrors. That portion of thermal-lensing which is not offset by the concave ends of the rod provides that the resonator operates as a stable resonator. An advantage of this type of resonator is that the laser beam optimally "fills" the gain-medium rod permitting optimum extraction of energy from the rod.
Medical lasers are usually operated over a wide range of power levels. Consequently, the thermally-induced positive dioptric power of the gain-medium varies. An advantage of the flat-flat resonator is that the resonator remains in a stable regime for a wide range of thermally-induced dioptric powers.
A characteristic of this type of resonator, however, is that as the thermally-induced dioptric-power increases, the width at the resonator mirrors of a laser-beam circulating in the resonator decreases. This increases the possibility of laser-damage to the resonator mirrors. In particular, it has been observed that the maximally-reflecting mirror is more susceptible to laser-damage than the output-coupling mirror. Accordingly, there is a need for a laser-resonator design which has the multimode operating characteristics and gain-medium energy extracting characteristics of the symmetrical flat-flat resonator but which is not limited in output-power by the laser-damage resistance characteristics of the maximally reflecting mirror.